A service brake actuator applies the wheel brake device more and more as the pressure applied increases, whereas a spring-type brake actuator applies the wheel brake device with the same spring force without any application of pressure. Applying pressure to a spring-type brake actuator tensions its spring and therefore counteracts the application of the wheel brake device. Spring-type brake actuators are therefore used as parking brake, for example.
The parking brake on many of these vehicles is also used as auxiliary brake whilst underway, should the service brake or parts of the service brake fail.
Such utility vehicles have at least one front axle and at least one rear axle.
The service brake part of such brake systems is designed with at least two brake circuits, one service brake circuit usually being assigned to at least one front axle and the second circuit being assigned to at least one rear axle.
In many utility vehicles spring-type brake actuators are used on more than one axle, in order to allow an adequate braking function of the parking brake, especially in cases of auxiliary braking.
Many of these vehicle have an air suspension ride height adjustment for raising and lowering the entire vehicle. As a result the kinematic wheelbases may vary according to the geometry of the axle suspension.
One disadvantage of such a brake system is that when stationary with spring-type brake actuators engaged the wheels thus equipped are firmly locked to the axle, so that in the event of a ride height adjustment, for example, strains can occur in the stationary vehicle, since the wheelbases are fixed. All spring-type brake actuators cannot be released simultaneously, however, since unwanted vehicle movements might then occur.